Oil and natural gas industry products, like many other electronics products, utilize a wide range of electrical and electromechanical machines and products, many of which are installed in hazardous locations including explosive atmospheric environments. A potentially explosive atmosphere exists when a mixture of air gases, vapors, mists, or dusts combine in a manner that may ignite under certain operating conditions. Equipment and protective systems intended for use in potentially explosive atmospheres (e.g., ATEX/IEC) cover a range of products, including those utilized in, for example, fixed offshore platforms, petrochemical plants, mines, and flour mills, amongst others. Products implemented in such conditions are defined by ATEX standards.
Many of these products are battery operated and support main power configurations as well as battery power. In certain cases, especially for products which need to preserve functionality and/or data, a back-up battery option may be required to ensure on-process changing of a main-battery while continuing operations during a main power outage. In some situations, two batteries may be utilized. That is, one battery may serve as a main battery and a second battery may function as a back-up battery for the main battery.
In most prior art designs, a main battery is assisted by a back-up battery to provide uninterrupted power to the load. In such situations, a diode-based scheme is primarily utilized because such a scheme in general is relatively easy to implement. A general observation can be made that battery monitoring/management devices available in the market consume very high currents for their own operations. Conserving this leaky current becomes very critical in battery-operated devices, which are expected to last in some cases for many years. The use of complex devices for critical functions such as dealing with two or more power sources makes such devices difficult to qualify, for example, for Ex (Explosive Protection) requirements (e.g., hazardous area certifications). In such cases, a general recommendation is to utilize simple devices such as a diode, FET (Field Effect Transistor), etc.
FIG. 1 illustrates a prior art schematic diagram depicting a standard main and back-up battery circuit 10. The configuration of circuit 10 shown in FIG. 1 is based on a diode arrangement. In the circuit arrangement shown in FIG. 1, a main battery 12 is electrically connected to ground 13 and a diode 14. A back-up battery 16 is electrically connected to a diode 18, which in turn is electrically connected to the diode 14 and to a load 20. Diode-based solutions such as circuit 10 depicted in FIG. 1 come with a built-in disadvantage because in such devices, the discharge rate is low and the life of the main battery 12 is expected to be long (e.g., at least several years). This issue is particularly evident when both batteries are of the same type. In some arrangements, the diodes 14 and 18 can form a DIODE OR network or circuit. Such a DIODE OR network or circuit can be utilized to isolate two or more voltage sources and also in some implementations to derive a simple Boolean logic function.
This type of circuit is subject to the following operations. First, as the main battery discharges and the voltage falls below the voltage of the back-up battery 16, the back-up battery 16 will kick in through a DIODE OR network. Then, as the back-up battery 16 discharges and the voltage falls below that of the main battery 12, the main battery 12 can kick-in through the DIODE OR network. These operations can happen in a back and forth manner and the order at which this occurs depends very much on several factors such as discharge rate, battery ambient temperature, the battery technology, and so forth.
FIG. 2 illustrates prior art graphs 22 and 24, respectively, depicting data indicative of short-term discharge characteristics and long-term discharge characteristics of the example circuit 10 shown in FIG. 1. As a result of the operations described above, the back-up battery 16 begins losing back-up energy along with that of the main battery 12. However, the design intent is to preserve the back-up energy to supply sufficient energy in the absence of main energy. Thus, there may be a state where back-up battery 16 is not in any better state than the main battery 12 and not be in a position to take over from the main battery 12 during an unexpected power outage from the main battery 12.
FIG. 3 illustrates an example graph 30 depicting the battery characteristics of a sample Thionyl Chloride Lithium Battery utilized in an experimental embodiment. Note that these are 3.6V non-rechargeable batteries. The discharge of these batteries is very steep during the last part of the cycle and this is where above-mentioned issues become a concern. The voltage and/or discharge profile varies with temperature also.